INTERNATIONAL - The 2.4 trillion yen ($21 billion) initial public offering of SoftBank Group Corp.’s phone unit is aimed at attracting retail investors to maximize its value. No surprise then, that the technology company and underwriter Mizuho Securities Co. are turning to a smartphone-based brokerage to sell shares.

The stock-trading app they’ll use, One Tap BUY, also happens to be backed by SoftBank and Mizuho. The plan is to make it easier for individual investors to subscribe to the IPO by splitting the lots. That would let people buy one share at the indicated price of 1,500 yen apiece, instead of the usual minimum block of 100 shares for 150,000 yen, according to the prospectus.

The public debut of the wireless unit, a new entity named SoftBank Corp., is part of founder Masayoshi Son’s grand plan to turn the parent into a global investment company through the $100 billion Vision Fund. Retail investors will be a key part of the offering, which could exceed the IPO of former national carrier Nippon Telegraph & Telephone Corp. in 1987. The underwriters plan to sell about 2 trillion yen worth of shares to Japanese individual investors.

“People can buy into the shares with the money in their wallets,” said Katsumi Udagawa, manager of research at Ichiyoshi Securities Co. “Even people who aren’t used to investing in stocks, or SoftBank smartphone users, can become stockholders.”

One Tap BUY is rolling out a new app dedicated to IPOs, starting with the SoftBank offering, according to its website. Representatives at SoftBank and Mizuho declined to comment on how much they plan to sell via the app.

Of the Japanese brokers underwriting the IPO, Nomura Holdings Inc. plans to sell the largest portion, followed by Daiwa Securities Group Inc. and Mizuho, Bloomberg News reported earlier this month. Mizuho’s share of that is 410 billion yen, although it’s not clear how much of that is targeted to be sold via the app.

As part of the efforts to attract more investors, Nomura, Mizuho and other underwriters have embarked on a television marketing campaign, seeking to appeal to a wide demographic. Investors who purchase shares using One Tap BUY won’t get voting rights, but will be eligible to receive dividends. SoftBank owns a 48 percent stake in the smartphone brokerage, while Mizuho holds a 12 percent stake, a regulatory disclosure shows.

“This could pave the way to expanding the number of individual investors, by getting depositors to put some of their money into the stock market,” Udagawa said.